PROJECT SUMMARY Inflammatory bowel disease (IBD) and colonic malignancy are heterogeneous disease states that result from a complex interplay of host genetic and environmental factors. It is becoming increasingly clear that early events in development of the colonic microbiota influence host immunity, nutrition, and susceptibility to disease. Bacteroides fragilis comprises up to 2.5% of the human microbiota, and is often acquired within the first month of life. A subspecies of Bacteroides fragilis termed enterotoxigenic B. fragilis (ETBF) releases B. fragilis toxin (BFT), a zinc-dependent metalloprotease that causes a pro-inflammatory injury of the intestinal epithelium. ETBF has been implicated in the pathogenesis of IBD, colon tumorigenesis, acute diarrhea, and undernutrition in children. ETBF colonizes up to 20% of asymptomatic humans, suggesting that these individuals may incur an underappreciated long-term health risk from chronic carriage. We have demonstrated that competition for the B. fragilis niche within the colon is governed by strain-specific determinants including the Type VI bacterial secretion system. Further, the acquisition of protective strains of NTBF that restrict ETBF acquisition blunt the toxic effects of ETBF and thereby mitigate disease. The primary goal of this proposal is to examine neonatal acquisition of ETBF as a determinant of host susceptibility to disease. Through a comprehensive analysis of the genetic determinants of ETBF colonic niche establishment and the mechanisms by which BFT is expressed and released to act upon host cells, this study will define fundamental mechanisms that underlie ETBF-mediated disease. These studies will benefit from the use of a novel model of B. fragilis vertical transmission in which the temporal and genetic determinants of initial niche colonization by B. fragilis is examined in neonatal mice. It is anticipated that these studies will shed light on strategic opportunities for genetically informed probiotic-based approaches to modulate colonic disease through strain-specific niche competition, precluding the deleterious acquisition of ETBF that renders a host susceptible to disease.